Most new drug candidates generated during in vitro screening turn out to be invalid after time-consuming and costly testing in animal models. Therefore, there is an urgent need for development of noninvasive, real-time, sensitive, and cost-effective tools with high throughput, for monitoring and early detection of drug efficacy in vivo.
Optical molecular imaging provides many advantages over other imaging modalities, including high sensitivity and safe detection with non-radioactive materials, using readily available instruments, at moderate cost. Peptide-based molecular beacons, otherwise known as protease activatable optical probes or imaging agents, have enabled in vivo imaging of protease activity and demonstrated promising results in the field of protease research and protease-targeted drug development.
Proteases are known as exceptionally critical signaling proteins that are involved in numerous processes, such as inflammation, as well as cancer, neurological disorders, and cardiovascular diseases. Considerable efforts have been made to identify the role of certain proteases in biological processes and to screen specific molecules that can regulate protease expression. There are known experimental methods based on the use of protease reporters or molecular beacons. However, they are limited to in vitro applications.
With the development of hydrophilic near-infrared (NIR) dyes and quenchers, it is now possible to use conventional molecular beacon constructs as in vivo imaging agents. These imaging agents are optically silent (quenched) in their native state and are activated in the presence of a specific protease, thereby generating an NIR fluorescence signal. However, the inherent instability, short half-life, long activation time in vivo and/or nonspecific activation of peptides and small compounds are still major obstacles to their in vivo application via systemic administration.
In view of the foregoing, there currently exists an unmet need for a fast acting and/or extended-use activatable imaging agent which can permit real-time video imaging of protease expression in vivo.